Hydraulic servo with integral equalization



Sept. 27, 1955 R. RISHEL ETAL HYDRAULIC SERVO WITH INTEGRAL EQUALIZATION 2 Sheets-Sheet 1 Filed May '7, 1954 .mmmlil i .lu gwn Julius Sept. 27, 1955 R. RISHEL ETAL 2,718,877

HYDRAULIC SERVO WITH INTEGRAL EQUALIZATION Filed y 7, 1954 2 Sheets-Sheet 2 v @WWM United States Patent" "6) HYDRAULIC SERVO WITH INTEGRAL EQUALIZATION Robert L. RisheLBerkeley, and Augustus P. Henry, Los Angeles, Calif., assignors to Northrop Aircraft, Inc., Hawthorne, Califi, a corporation of California Application May 7, H54, Serial No. 428,190

9 Claims. (Cl. 121-41) This invention relates to hydraulic servo mechanisms and more particularly to a hydraulic servo mechanismincludinga main valve controlling fluid How to and from an actuatoigthe valve being displaced fromits neutralposition by a function of the error of the system of which the mechanism constitutes a. component part.

It: is well known that av hydraulic system frequently becomes dynamically unstable especially when operating under controlled conditions For example, if stringent performance specifications of a hydraulic system are to be met a specified speed of response of the servo ,mechanism associated therewith is required. Inorder that the system may meet specific performance specifications it is common practice to increase the gain of the servo mechanism. Increasing the gain of a servo mechanism associated with such a high performance hydraulic. system invariably results insystem instability before. the desired speed of'response is achieved.

Instability in a hydraulic system is readily recognized by resonant frequenciesor other. disturbing. phenomena occurringwithin the system. These. resonantfrequencies. may, feed energy into acontrolled. systemrin suchamanner that disastrous results to the systemwill follow. The aforementioned. frequencies may also be.- of the. same. magnitude as oscillations or vibrations of.,mechanicalt devices, flutter frequencies of 'an,air.craft,,etc. withwhich the hydraulic system isassociated. In the aboveflcases dynamic couplingmay occur. betweenthe; resonant frequencies of the hydraulic. system andthe. oscillations, vibrations, or flutter frequencies. Such, a. coupling and the resulting'resonant frequencies may well be of. such. a magnitude that disastrous. resultsto both. the hydraulic system and associated structure willfollow.

As previously mentioned the gain of a servo, mech-v anism may be increased in:an attemptto have ahydraulic system meet certain, performance. specifications... If. the: gain of a conventional servo mechanism,.for. example of. the type shown and disclosed in; U. S.. Ratent No; 74 619,304, is materially increased," that is; beyond. its. designed1operatingcapacity, incipientinstability willloccur. in the system with which the mechanism is associated; In. such a conventional mechanism. the. relative position, of the valve spool, with respect. to its operating sleeve, is directly, proportional to the error? of the system, with. which the mechanism is associated. In other. words it may be said that the, valve spool is positioned by the error of the system.

Experimental tests have shown that the gain of a. conventional servo mechanism maybe increased, that is increased to the extent that'it may be made compatible with stringent performance requirements of'a hydraulic system with which his associated, if the valve'controllin-g fluid fiow' to an associatedactuatorormotor is displacedfrom its neutral position by a' function ofthe errorof" the system rather than by the-error *of 'the system itself.

To accomplish-the above objective the servo mechan-ism of the instant inventionincorporatesa; manual operable auxiliary valve, a-mainufiuid control valve', andian actuating piston all mountedand operating in a common housing; The main valve controls fluid flow to and from the piston chamber, this valve is displaced fromits neutral position byia function of the error of the system by 'novel means and .in a manner that will be presently explained.

Accordingly it is an object of the instant inventionxto provide a hydraulic servo mechanism including an actuator and a fluid controlvalve and in which the valve is displaced from, its neutral position by a particular function of the error of thesystem with which the mechanism is associated.

Another'object isv to provide a hydraulic servo mechanism including an actuator and a fluid control'valve and in which the valve will not be influenced by. actuator output; In other wordsthe. position ofthe controlvalve will not be influenced by external forces actingon. the actuator.

These and other objectstwill become more apparent from the following description and drawingsv in which the reference characters denote like parts throughout the several views. It is to be expressely understood, however, that the drawings are for the purpose of illustration only and not a definition of the limits of. the invention, reference being hadfor this purpose to the. appended claims.

Figure l is a. schematic sectional view of a hydraulic servo mechanism in whichtthe fluid control valve is displaced as. disclosedin the instant'invention.

Figure 2 is a view similar to Figure l but showing another embodiment. of a servo-mechanism asdisclosed herein.

Certain terms used throughout the specification and appended'claims are defined as follows:

Gain as used in the present disclosure in connection with a servo mechanism including an actuator and a fluid control valve, is defined as the ratio of valve displacement (i. e., travel distance of the. valve away from its neutralposition) with respect to the rate of fluid flow to theactuator.

Error as used in the present disclosure in connection with aservo mechanism including an actuator and fluid. control valve, is definedas the valves relative positionwith respects. to the actuator housing. Instantaneous error refers to the valves-relative position with respects to the actuator housing ataparticular instant. The. error ofiahydraulic system may be. expressed'by the following formula:

- x0 equals the-actuators output (actuators displacement with-respectto the fixed' reference--point)- Kefrringznow 'torrthezdrawings;. Figure 1: shows a servo mechanism assembly; 111 comprising-a housm ilili The housing has parallel; spaced, bores, 18 and. 19; formed therein in:whi'ch antauxiliary-valve 14; almain-fiuid con trolf V81V6z16, and; a differential? area piston type; actuator 17- are operationally mounted; llhe auxiliary and control valves, are: positioned inthe"; bore. 1-8 located: in:- the upper; portion. of thehousi'ngwhile the: actuator operates: in the. bore- 1i9.-in. the lower; portion of the" housing;- The bone- 19;:is closed at: one. end by wallportionsmfthe: housing andpartiallyat its other end bya: gland 21'. The:bore 18 isxcontinuous;throughout the extentsof-thedi-ousing 12, it: is,. however, partially closed. at its ends by; glands 23: and; 24 having annular disk portions: 2'fiiandi30; respec-;- tively. i

The. actuator 17$ i suconstructed so that: its circulanface. 261 is twicezthe areaiofaitsannular: face 27.. Thisficona struction is for? a:, purpose i which will. benapparen'ttlater:

An end of the actuator extends through the gland 21 and is provided with an eye 22 whereby it may be secured to stationary structure 15. Accordingly the housing 12 becomes the output member of the assembly 11. Structure to be moved by the assembly 11 may be attached to the housing and for this purpose a drilled hole 28 is provided in a suitable portion thereof.

The auxiliary valve 14 comprises a spool 29 and shank 31. The shank extends through the aperture in the gland 23 and is provided with a connecting end 32, the latter providing means whereby the spool may be moved between its various operational positions. The valve is constructed so that the circular face 33 of the spool 29 is twice the area of its annular face 34. Secured to the disk 25, so that the shank 31 passes through an aperture in its bottom portion, is a cup shaped member 36 adapted to hold hydraulic fluid. Extending diametrically through the shank 31 is a rectangular integrating slot 37. This slot extends axially of the shank a distance equal to or slightly greater than the thickness of the disk portion 25. The slot 37 connects with a longitudinal bore 38 located internally of the auxiliary valve and which extends to the surface 33. A port 39 is provided in the side Wall of the member 36 whereby the aforementioned fluid may enter the chamber 41 defined by the member and associated parts.

The main fluid control valve comprises cylindrical spool portions 42, 43, 44 and 46 separated by connecting portions 47, 48 and 49, respectively, of less diameter than the spool portions. The control valve is also provided with a shank 51 extending axially from the spool portion 46. The shank 51 passes through and remains in contact with the wall surfaces of the aperture in the disk portion 30 during all operational positions of the valve 16. The axial movement of the valve 16 is limited by stop members 40 extending from the wall of the bore 18. The diameter of shanks 31 and 51 are identical accordingly the area of the annular faces 34 and 52 of the auxiliary valve and control valve, respectively, are equal as the two valves operate in the common bore 18. Secured to the plate 30 and surrounding the shank 51 is an imperforate cup shaped member 53 adapted to hold fluid at the same pressure as fluid contained in the chamber 41. A port 54 allows fluid to enter the chamber 56 defined by the member 53 and associated parts. Extending diametrically through the connecting portion 47 is a bore 57 which connects with a longitudinal bore 58, the latter extending to the face 59 of the spool portion 42.

With the valves 14 and 16 positioned substantially as shown in Figure 1, chambers 70, 72, 74, 78, 79 and 83 are defined by the valves in conjunction with the bore 18 and other associated parts. The chambers 78, 79 and 83 constitute variable volume chambers while the other chambers are of constant volume. Cylindrical surfaces of the auxiliary valve, the control valve, and portions of the actuator which contact the walls of the bores 18 and 19, respectively, as well as the walls of the bores themselves are lapped so that a minimum of fluid will leak past these mating surfaces. The shanks 31 and 51 and the walls defining the apertures in the glands 23 and 24 are also lapped to insure minimum fluid leakage.

The control and auxiliary valves are shown in their neutral positions in Figure 1, that is no movement of the housing 12 will occur with the valves in the relative position as shown. Three circumferential grooves are provided in the wall of the bore 18, these grooves bear a definite relation with respect to the control valve when the latter is in its neutral position. A first groove 61 has one of its side faces in line with the side face 62 of the spool 42, its other side face overlays the cylindrical surface of this spool. A second groove 63 has one of its side faces in line with the side face 64 of the spool 43, its other side face overlays the cylindrical surface of this spool. A third groove 66 has its side faces in line with respective side faces 67 and 68 of the spool 44.

The construction of the servo assembly is completed by various passages and ports, formed in the housing 12, which allow fluid to communicate with the various chambers provided therein. A first connecting passage 69 pro vides fluid communication between the chamber 72 and the chamber 73, the latter being located in the bore 19 adjacent the annular face of the actuator 17. A second connecting passage 71 provides fluid communication between the groove 66 and chamber 76, the latter being located in the bore 19 adjacent the circular face of the actuator 17. An auxiliary passage 77 and lateral passages 77a provides means whereby fluid from an external constant pressure source (not shown) may flow to chambers 78 and 79 and the groove 61. An outlet passage 81 and lateral passages 81a provide means whereby fluid may flow from the groove 63 and chamber 74 to a reservoir or the like (not shown). An inlet passage 82 provides means whereby fluid may flow from a fluid source (not shown) to the chamber 72.

During the operation of the assembly 11 fluid at the maximum pressure occurring in the hydraulic system with which the assembly 11 may be associated enters the chamber 72 through the inlet passage 82. The aforementioned maximum fluid pressure is designated as PM (Figure 1) and for purposes of illustration is assumed to be 3000 p. s. i. Inasmuch as axial movement of the control valve is limited by the stop members 40 during the operation of the assembly, it will be apparent that chambers 72 and 73 are subject to fluid at the pressure PM throughout the operation of the assembly. Accordingly for a balanced condition of the actuator 17, with respect to the housing 12, chamber 76 must be subject to fluid at a pressure of one half PM (PM/2). This is necessary as the area of the faces 26 and 27 of the actuator have a ratio of two to one, respectively. Relative positions of the actuator 17, with respect to the housing 12, are controlled by metering fluid to and from the chamber 76.

Each of the chambers 78 and 79 and the groove 61 are subject to fluid at a constant pressure PC, this latter fluid passing through the auxiliary passages 77 and 77a. As the valve 16 is free to move fluid at a pressure of one-half P will occupy chamber 83 if the valve 16 is to remain balanced. The pressure of fluid in chamber 83 is indicated in Figure 1 as PR. Fluid at return system pressure Px is ported to the chambers 41 and 56, this latter fluid also communicates with the chamber 74 and groove 63 through return passages 81 and 81a. The return system pressure PX is assumed to be atmospheric.

A more complete understanding of the servo assembly will be forthcoming from the following discussion of its operation.

In the instant servo assembly the auxiliary valve 14 is first displaced axially a given amount from its neutral position, for example it may be displaced a distance X1 to the right. This movement of valve 14 is initially transmitted to the control valve 16 through the solid column of fluid in chamber 83. Accordingly the valve 16 is moved an axial distance equal to the axial movement (X1) of the valve 14. If this was the only movement imparted to the control valve it would be positioned by the error of the system as is the case in connection with conventional valves.

The above movement however, is not the only movement imparted to the control valve as fluid contained in the chamber 83 is augmented as the error valve is moved to the right. Fluid flows from the chamber 78, via the integrating slot 37 and the bore 38, into the chamber 83. This flow takes place as the integrating slot 37 has been uncovered and the fluid pressure P0 is twice the fluid pressure PR. This flow of fluid results in the control valve moving through a greater axial distance than the auxiliary valve, or a distance in excess of X1. Accordingly the displacement of the valve 16, in the discussion so far and before the housing 12 and actuator 17 has-reached theirnew equilibrium positions,

43 uncovers a portion ofthe groove 63. This allows fluid. at a pressure 'PR to flow from the chamber 83 through the passage 58, bore 57, chamber 70, groove 63, and, return passages .81 and-81a to the reservoir (not shown) at the pressure PX. This flow of fluid takes place at a rate proportional to control valve displacement and tends to return the control valve toits neutral position, due to fluid in chamber 79 at pressure Po acting on the end surface 52 of the spool portion 46.

Simultaneously as the control valve 16 is displaced from its. neutral position in .themanner described above, the housing 12 is being moved with respect to the actuator 17 in the normal fashion. Fluid at the pressure PM is metered into ithe actuator chamber 76. Inasmuch as the area of the face 26 is twice that of the annular face 27, the latter also subject to fluid pressure PM, the housing 12 is moved to the right. This movement continues until the control and auxiliary valves are simultaneously returned to their neutral positions and system equilibrium is reestablished. Thus it is seen a hydraulic valve is provided meeting the aforementioned objects.

If the auxiliary valve 14 is moved .to the left, for example a distance X1, the sequence-of control valve and actuator-housing movements will be substantially the same as thosedescribed above exceptthat the latter two elementswill move to. the left. With fluid at return system pressure (PIX) in chamber 41 the pressure drop across the slot 37' will be constant (Pa/2) regardless of the direction in which the auxiliary valve is displaced. Accordingly as the. auxiliary valve is moved to the left, fluid at pressure P in thechamber 79 acting on the face 52 of the spool portion 46 will cause the control valve to follow. 'During this movement of the control valve fluid from the chamber 83 will flow through the bores 38 and 37 and into the chamber 41. From the chamber 41 this fluid will be exhausted to the return system at the pressure: PX. This last flow of fluid allows the control valve to travel a distance in excess of that initiated by the movement of the auxiliary valve 14. Simultaneously fluid. flows from, the groove 61 through thebores 57 and 58 to increase the quantity of fluid in chamber 83, this latter flow of fluid moves the control valve to the right and augments its return to its neutral position. The above movement of the main valve to the left uncovers. a portion of the groove 66 and allows fluid to flow from the actuatorlchamber 7.6. to the return system. Accordingly the housing 12 is moved to the left until' the control and auxiliary valves are returned to their'lneutral positions and system equilibrium is reestablished.

In. both of the above illustrations the auxiliary valve is also returned to its neutral position due to movement of the housing 12. .In this position the disk portion of the gland 23 spans the slot 337 effectively arresting fluid flow therethrough.

The present valve construction constitutes a hydraulic analog of a standard electrical R-C lead network as generally used to increase the gain .of a conventional servo mechanism. For example, a standard R-.C lead network may be inserted in series with an electro-mechanical transducer which positions the control valve of a conventional hydraulic servo mechanism. The R-C lead network provides a positive phase shift through a given frequency region which renders the system dynamically stable and at the same time allows a very high gain to be achieved. Inasmuch as the R-C electrical lead neture 1 is shown. of the assembly 111 is deemed necessary. The same workis. widely used in-this respect and its design-application so thoroughlydescribed in standard texts. :no further description is deemed necessary herein. 1 The valve construction disclosed herein. acts in a similar manner, it renders a hydraulic system dynamically stable and at the same time permits a very high gain to be achieved therein.

Referring to Figure 2, a servo mechanism 111 similar in construction and operation to the assembly 11 of Fig- Accordingly only a brief description numerals have been used to designate identical corresponding parts in the two assemblies, however, subscripts have been added in the case of the latter assembly in some instances.

The auxiliary valves 14 in each embodiment are identical, the control valve 161 of assembly 111, however, constitutes a fourway valve as compared with the three way valve of assembly 11. Accordingly both of the sides 261 and 271 of the actuator 171 are of equal area and are subject to the same fluid pressure at such time as the control valve 161 assumes its neutral position as shown in Figure 2. The control valve 161 comprises cylindrical spool portions '84, 86, and 87 separated by connecting portions 88 and 89 which are of less diameter than the spool portions. Extending from the spool portion 87 is a cylindrical shank 511, having the same diameter as the shank 31 ofthe valve 14. The valves 14 and 161 together with the bore 18 defines chambers 78, 79, 83, 91, and 92. Cylindrical grooves 93, 94, and 96 are formed in the cylindrical wall of the bore 18 and bear the relation shown inFigure 2 at such times as the control valve 161 is in its neutral position. A rectangular slot 97 extends diametrically through the shank 511 so that its side edges are in line with side faces of the disk portion 30 of the gland 24 when the control valve is in its neutral position. Extending internally of the valve 161, between the slot 97 and the opposite end of the valve, is a longitudinal bore 98.

System fluid at a maximum pressure PM enters the housing 121 and groove 94 through the inlet passage 821. With the control valve inits neutral position a predetermined amount of fluid will leak past the spool portion 86 and subject the chambers 731, 761, 91, and 92 to a fluid pressure approximately one half the system pressure or PM/Z. This fluid in turn leaks past the spool portions 84 and 87 and into the grooves 93 and 96 from whence it flows to return at the pressure PX through the return passages 811 and 81m. Under the above conditionsthe various parts of the assembly 11 are in equilibrium and no movement of the actuator occurs.

If the auxiliary valve 14- is moved a given distance X1 to the right the control valve 161 will be moved an equal distance to the right by the column of fluid in chamber 83. At the same time fluid will flow from the chamber 78 to the chamber 83, via the integrating slot 37 and the bore 38, tending to displace the control valve 161 to the right a greater distance than X1. Simultaneous with these two movements of the control valve fluid will also flow from the chamber 83 to the chamber 56, via the bore 98 and slot 97, effecting a partial return of the-control valve to the left and its neutral position. Thus it is seen thatthe control valve 161 is displaced from its neutral position by a function of the error of the system in a similar manner as described in detail in connection with the embodiment shown in Figure l. A movement of the auxiliary valve 14 to the left will result in a similar movement of the valve 161 to the left in a manner that is believed to be apparent and further explanation is deemed unnecessary.

In the case where the auxiliary and control valves 14 and 161are moved to the right fluid at system pressure (PM) will. flow to chamber 761 while the chamber 731 will be vented to return pressure (P51). Accordingly the housing 121 will: be'moved" to the right until the relative position of the valves and housing assume the same relationship as shown in Figure 2, at this timethe system will again be in equilibrium.

V A certain amount of fluid flow occursin connection with either of the valves 16 or 161 at such time as they are in their neutral positions. In'Figure l a predetermined fluid flow occurs from the inletpassage 82 to the outlet passage 81 past the spool portion 44. A pressure drop. occurs as this fluid flows past the spool portion 44 so that the pressure P2 of fluid in chamber 76 is one half the pressure P1 of fluid in chamber 73. In Figure 2 fluid fiow occurs from inlet passage 82 to each lateral outlet passage 81:11 past the spool 86 causing a predetermined pressure drop whereby the fluid pressure P3 and P4 present in chambers 731 and 761, respectively, are equal and approximately one half of the maximum system pressure PM.

Although the valve and actuator combinations are shown schematically it is conventional practice to have the valve elements 14, 16, and 16 operate in cylindrical sleeves pressed in the bores 18. If sleeves are utilized the cylindrical grooves will be replaced by radial bores extending through the aforementioned sleeve. Also the control valves 16 and 161 may be utilized to control the flow of fluid to a rotary or any other type of motor. Accordingly it will be appreciated that various omissions, substitutions, and changes may be made by those skilled in the art without departing from the teachings of the invention.

While in order to comply with the statute, the invention has been described in language more or less specific as to structural features, it is to be understood that the invention is not limited to the specific features shown, but that the means and construction herein disclosed comprises preferred forms of putting the invention into effect, and the invention is therefore claimed in any of its forms or modifications within the legitimate and valid scope of the appended claims.

What is claimed is:

1. In a hydraulic system a valve assembly, comprising: a housing having a closed bore therein; wall portions of said housing defining an inlet, an outlet, and an auxiliary passage and first and second connecting passages each extending between said bore and external surfaces of said housing; an auxiliary valve and a control valve mounted in said bore for axial movement through respective predetermined ranges and being spaced with respect to each other to provide a variable volume chamber therebetween; a spool portion of said auxiliary valve and spaced spool portions of said control valve being in r fluidtight relation with the wall of said bore; said valves having neutral positions with respect to said housing from which said valves may be moved in either axial direction an equal amount before reaching the ends of their respective ranges; means whereby said auxiliary valve may be manually moved throughout its respective range; said control valve being free to move throughout its respective range when contacted by fluid under pressure; portions of said auxiliary valve defining an interior passage therein adapted to provide a continuous passage between said variable volume chamber and said outlet passage or said auxiliary passage according to the direction of movement of said auxiliary valve from its neutral position; means in said housing closing said interior passage when said auxiliary valve is in its neutral position; said control valve in the neutral position thereof cooperating with the wall of said bore to provide a continuous passage between said inlet passage and said first and second connecting passages and in all other positions throughout the respective range thereof various continuous passages between said inlet, outlet, and connecting passages.

2. Apparatus as set forth in claim 1: further characterized by portions of said control valve defining an interior passage therein adapted to provide a continuous s. passage between said variable volume chamber and said outlet passage or said auxiliary passage according to the direction of movement of said control valve from the neutral position thereof; and means in said housing adapted to. block fluid flow through said interior passage in said control valve when the latter is in said neutral position thereof.

3. Apparatus valve as set forth in claim 2; in which said various passages defined by said control valve provide a continuous passage between said inlet passage and each of said connecting passages when said control Valve is moved in one axial direction from the neutral position thereof and when moved in the opposite axial direction a continuous passage between said inlet and first connecting passages and between said outlet and second connecting passages, respectively.

4. Apparatus valve as set forth in claim 2: in which said various passages defined by said control valve provide continuous passages between said inlet and second connecting passages and between said outlet and first connecting passages when said control valve is moved in one axial direction from the neutral position thereof and when moved in the other axial direction continuous passages between said inlet and first connecting passages and between said outlet and second connecting passages.

5. In a hydraulic system the combination, comprising: a fluid motor including a housing having a piston operationally mounted in a cylinder in said housing; a bore in said housing extending parallel to the axis of said cylinder; means closing the ends of said bore; an auxiliary valve and a control valve mounted in said bore for axial movement thereinthrough respective ranges and being spaced with respect to each other whereby a variable volume chamber is provided therebetween; a major portion of said auxiliary valve and spaced portions of said control valve being in fluidtight relation with the wall of said bore; said valves having neutral positions with respect to said housing in which each valve is located at the midpoint of its respective range; wall portions of said housing defining an inlet, an outlet, and an auxiliary passage extending between said bore and an external surface of said housing and adapted to be connected to a source of fluid at super-atmospheric pressure, a fluid reservoir, and a source of fluid at constant pressure, respectively;

- first and second connecting passages extending between said bore and opposite ends of said cylinder; means attached to said auxiliary valve whereby the latter may be manually moved throughout its respective range; a passage in said auxiliary valve adapted to provide fluid communication between said variable volume chamber and said outlet or said auxiliary passage according to the direction of displacement of said auxiliary valve from its neutral position; said control valve in its neutral position cooperating with the wall of said bore to define fluid passages between said inlet, outlet, and connecting passages whereby fluid may contact opposite faces of said piston to exert balanced forces thereon; said control valve being movable from its neutral position to other positions throughout its range in response to movements of said auxi liary valve and fluid flow to or from said variable volume chamber; said control valve when in said other positions defining fluid passages between said inlet, outlet, and connecting passages whereby unbalanced fluid forces contact opposite faces of said piston; and means for attaching said piston and said housing to stationary and movable structure, respectively.

6. Apparatus as set forth in claim 5; in which said means closing the ends of said bore constitute an integral portion of said auxiliary valve extending from one end of said bore and cooperating with a first gland and an integral shank portion of said control valve extending from the other end of said bore and cooperating with a second gland; said first gland closing the passage in said auxiliary valve when the latter is in its neutral position.

7. Apparatus as set forth in claim 6; including a passage in said control valve adapted to provide fluid communication between said variable volume chamber and said outlet passage or said auxiliary passage according to the direction of movement of said control valve from the internal passage extending between an external surface of said shank thereof and the opposite end of said control valve and said means blocking fluid flow through the passage in said control valve constituting said second neutral position thereof; and means in said casing adapted 5 gland.

References Cited in the file of this patent UNITED STATES PATENTS 2,225,321 Schwendner Dec. 17, 1940 FOREIGN PATENTS 694,315 Great Britain July 15, 1953 

